This application claims the priority of Korean Patent Application No. 10-2004-0089162, filed on Nov. 4, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a memory device using a phase change material and a method of driving the same, and more particularly, a multibit phase change memory device and a method of driving the same.
2. Description of the Related Art
In recent years, a nonvolatile memory device retaining stored data even with supplied power cut off has been dramatically developed in technology along with the demand increase of portable personal assistants. A flash memory device as a representative nonvolatile memory device is substantially involved in most of the markets for nonvolatile memory devices with the advantage of low costs of production based on silicon processing. However, the flash memory device has drawbacks of requiring a relatively high voltage for data storage and limiting the number of repeated data storage. Accordingly, efforts to overcome the drawbacks for next generation nonvolatile memory devices are actively studied.
The next generation nonvolatile memory devices are roughly classified into two types, i.e., capacitor type memory device and resistor type memory device. A representative example of the capacitor memory device is a ferroelectric memory device using ferroelectric material, and the ferroelectric memory device reads stored data types from the polarization direction of the ferroelectric capacitor. A ferroelectric oxide material is mostly used for the ferroelectric memory device, but recently, the nonvolatile memory device using a ferroelectric organic material is actively researched.
Representative examples of the resistor type memory device are a magnetic tunnel junction memory device and a phase change memory device. The magnetic tunnel junction memory device, namely “magnetic RAM: MRAM”, has a structure in which a very thin insulating layer is inserted between two magnetic material layers, and operates to store data by controlling the spin polarization direction of the two magnetic material layers surrounding the insulating layer, and read stored data types from the magnitude of the tunnel current passing through the insulating layer, i.e., the resistance magnitude, in the cases that the spin polarization directions are same or different.
The phase change memory device, namely “phase-change RAM: PRAM”, uses the characteristics of the phase change material in which resistance magnitude is changed in accordance with the crystal structure of the material. The phase change memory device stores data by controlling the crystal structure of a phase change material with the appropriate selection of current or voltage application, and reads stored data types from the change of the resistance magnitudes in accordance with the crystal structure of the phase change material.
The various nonvolatile memory devices exemplified as above have their own advantages and disadvantages. For example, ferroelectric memory devices have a long history to be studied, and satisfy most of the functions required for a next generation nonvolatile memory device, but have difficulties in the fabrication processing of making devices further scaled. Therefore, present technology is limited to realize a higher integration of flash memory devices.
In MRAMs, it is reported that operation characteristics of the device may be deteriorated with the miniaturization (or scaling) of devices, and power consumption of the device is necessarily increased along with the integrated devices while the operation speed of the device is very high.
On the contrary, PRAMs can use chalcogenide metal alloy-based phase change material which has been widely used for optical data storage devices such as CD rewritable (CD-RW), digital versatile disk (DVD), and the like. Further, as the fabrication processing of the phase change memory devices well matches with the fabrication processing for typical silicon-based devices, the phase change memory devices can be implemented with an integration degree equal to or greater than that of DRAM. Of course, there still remains a subject to further reduce consumption power for operation than ever, but seeing the results of technology development for PRAMs, they are recently noted as one of the most important non-volatile memory devices for next generation enabling to replace the existing flash memory devices.
Further, another advantage of the PRAM is to allow a multibit memory device being capable of storing plural memories in addition to 0 or 1 in one single device structure. This is required that the crystal structure of a phase change material is maintained a certain state other than fully crystallized structure or fully amorphous structure, and the state can be given by applying an appropriate electric energy, i.e., supply of a predetermined current. It is reported that a value of an intermediate state other than 0 or 1 can be achieved by applying a predetermined current signal. If such a multibit memory device having the function is realized, it is expected that the PRAM will replace all the markets for flash memory devices limited in integration degrees, and significantly increase the integration degree of memory devices. Accordingly, they are highly expected as non-volatile memory devices for next generation portable digital personal assistants.
However, in order to substantially realize such a multibit phase change memory device, a method quite different from the conventional manufacture method for typical PRAMs must be selected. The reason can be explained as follows.
First, a phase change material used for the multibit phase change memory device must exhibit a linear characteristics depending on applied electrical or thermal energy when it is changed from amorphous structure to crystal structure, or from crystal structure to amorphous structure. If a phase change material not linearly varied in its characteristics is used, peripheral circuits with very complicated structures must be prepared to determine operation conditions during a storage or a reading operation of data in intermediate states to drive a device. However, the phase change material satisfying the characteristics has not be developed up to now, and research examples for the required characteristics have not been reported, either.
Secondly, it has not studied about a method of driving such a multibit phase change memory device, and it is expected to adopt a method of appropriately controlling a pulse-type electrical signal of a predetermined level. In specific, there are examples, such as a method of controlling an amount of the thermal energy actually applied to a phase change material by adjusting the absolute value (magnitude of an applied signal) of a pulse-type electrical signal, and a method of controlling an amount of the thermal energy actually applied to a phase change material by changing the pulse width (applied time) of a pulse-type electrical signal. The methods are closely related with a crystallization rate or amorphization rate of the phase change material, and unless a method of arbitrarily controlling the crystallization rate or amorphization rate if necessary is developed, a driving operation of the device is very difficult to realize.
In consideration of the reasons, it is required to develop a new phase change material, which has not been studied, in order to manufacture a multibit phase change memory device and successfully realize the operation characteristics, and to study and research the material in detail. Therefore, much time is expected to take until verifying stable operations of the multibit phase change memory device, based on the current technology for the devices.